Cerebral edema associated with malignant brain tumors often causes neurologic deficits and increased intracranial pressure, which contribute to the morbidity and mortality of the neoplasm. We have determined that the conditioned medium of glioblastoma-derived cell cultures contains a substance capable of increasing the vascular permeability in a bioassay which measures the induction of capillary permeability in normal skin. The elicitation of this response by conditioned medium correlates with the edema observed clinically in patients from which these tumor cell cultures were derived. This factor appears to be distinct from other known inducers of vascular permeability. We are now in the process of purifying and characterizing this factor. Our long range goal is to better understand the regulation and mechanism of action of this factor in order to minimize the clinical complications.